1. Field of the Invention
The present invention relates to a semiconductor memory device, e.g., a resistance-change memory including a memory cell formed by connecting a diode and variable-resistance element in series.
2. Description of the Related Art
Conventionally known, commercially available semiconductor memory devices such as a DRAM, SRAM, and flash memory use a MOSFET as a memory cell. As the speed of shrinking increases, therefore, it is becoming increase the dimensional accuracy at a ratio higher than that of shrinking. Accordingly, a large load is imposed on the lithography technique for forming these patterns, and this raises the manufacturing cost.
Recently, a resistance-change memory is attracting attention as a candidate for succeeding a semiconductor memory device using a MOSFET as a memory cell as described above (see, e.g., Jpn. PCT National Publication No. 2005-522045). The resistance-change memory herein mentioned includes a resistance-change memory in a narrow sense (resistive RAM [ReRAM]) that contains a transition metal oxide as a recording layer and stores the resistance state of the recording layer in a nonvolatile manner, and a phase-change memory (phase-change RAM [PCRAM]) that contains chalcogenide or the like as a recording layer and uses resistance information of a crystalline state (conductor) and amorphous state (insulator) of the recording layer.
A variable-resistance element of the resistance-change memory has two kinds of forms. One is called a bipolar element by which a high- or low-resistance state is set by switching the polarities of an application voltage. The other is called a unipolar element by which the high- and low-resistance states can be set by controlling the voltage and voltage application time without switching the polarities of an application voltage.
The unipolar element is favorable to implement a high-density memory cell array. Because in case of using the unipolar element, a cell array can be formed by overlaying a variable-resistance element and a rectification element such as a diode at the intersection of a bit line and word line, without using any transistor. In addition, in case memory cell arrays like this are three-dimensionally stacked, a large capacity can be achieved without increasing the cell array area because transistor is not included in memory cell.
In the unipolar ReRAM, data is programmed in a memory cell by applying a predetermined voltage to the variable-resistance element for a short time. This changes the variable-resistance element from the high-resistance state to the low-resistance state. This operation of changing the variable-resistance element from the high-resistance state to the low-resistance state will be called a setting operation hereinafter.
On the other hand, data is erased from a memory cell by applying, for a long time, a predetermined voltage lower than that of the setting operation to the variable-resistance element in the low-resistance state after the setting operation. This changes the variable-resistance element from the low-resistance state to the high-resistance state. This operation of changing the variable-resistance element from the low-resistance state to the high-resistance state will be called a resetting operation hereinafter. The high-resistance state of a memory cell is, e.g., a stable state (reset state). In case of storing binary data, the data is programmed by the setting operation of changing the reset state to the low-resistance state.
In the resetting operation, a large current is supplied as a reset current to a memory cell. Therefore, the diode to be connected in series with the variable-resistance element should output a large current. When using a simple p-n junction diode as the diode, however, excess voltage can not be applied to an unselected variable-resistance element. That is, the junction breakdown occurs in the p-n junction of diode. This limits the output current.